White matter is made up of dense nerve fibers that connect neurons in different areas of the brain, forming networks. It gets its name from the light-colored electrical insulation, myelin, that coats the fibers. Overall, the tissue is essential for rapidly transmitting brain signals critical for learning and cognitive function.

In August, US authorities first acknowledged that American diplomats and their spouses stationed in Havana, Cuba, had been the targets of puzzling attacks for months. The attacks were carried out by unknown agents and for unknown reasons, using a completely baffling weaponry. The attacks were sometimes marked by bizarrely targeted and piercing noises or vibrations, but other times they were completely imperceptible.